The present invention relates to a method for making high thermal diffusivity boron nitride powders and the resulting powders.
Integrated circuit chips are steadily becoming smaller and more powerful. The current trend is to produce integrated chips which are steadily increasing in density and perform many more functions in a given period of time over predecessor chips. This results in an increase in the electrical current used by these integrated circuit chips. As a result, these integrated circuit chips generate more ohmic heat than the predecessor chips. Accordingly, heat management has become a primary concern in the development of electronic devices.
Typically, heat generating sources or devices, such as, integrated circuit chips, are mated with heat sinks to remove heat which is generated during their operation. However, thermal contact resistance between the source or device and the heat sink limits the effective heat removing capability of the heat sink. During assembly, it is common to apply a layer of thermally conductive grease, typically a silicone grease, or a layer of a thermally conductive organic wax to aid in creating a low thermal resistance path between the opposed mating surfaces of the heat source and the heat sink. Other thermally conductive materials are based upon the use of a binder, preferably a resin binder, such as, a silicone, a thermoplastic rubber, a urethane, an acrylic, or an epoxy, into which one or more thermally conductive fillers are distributed.
Typically, these fillers are one of two major types: thermally conductive, electrically insulative or thermally conductive, electrically conductive fillers. Aluminum oxide, magnesium oxide, zinc oxide, aluminum nitride, and boron nitride are the most often cited types of thermally conductive, electrically insulative fillers used in thermal products. Boron nitride is especially useful in that it has excellent heat transfer characteristics and is relatively inexpensive.
However, in order to achieve sufficient thermal conductivity with presently used fillers, such as boron nitride, it has been necessary to employ high loadings of filler in the binder (see, e.g., U.S. Pat. No. 5,898,009 to Shaffer et al. and U.S. Pat. No. 6,048,511 to Shaffer et al.). Thus, there is a need for thermally conductive filler materials which can be used at low loading levels to achieve sufficient thermal conductivity. The present invention is directed to overcoming this deficiency in the art.
The present invention relates to a method for making boron nitride powder having a thermal diffusivity of from about 0.14 cm2/s to about 0.20 cm2/s. This method involves pressing high purity, hexagonal boron nitride having an average platelet size of at least 2 microns into a compacted form, sintering the compacted form of boron nitride to form a sintered body, and crushing the sintered body under conditions effective to produce boron nitride powder having a thermal diffusivity of from about 0.14 cm2/s to about 0.20 cm2/s.
Another aspect of the present invention relates to boron nitride powder having a thermal diffusivity of from about 0.14 cm2/s to about 0.20 cm2/s.
The method of the present invention produces high thermal diffusivity boron nitride powder which will exhibit high thermal conductivity when used as a filler for thermal management applications, e.g., in composites, polymers, and fluids. The resulting high thermal diffusivity boron nitride powder can also be used in composite polymers and metals for friction applications. Moreover, the resulting high thermal diffusivity boron nitride powder can be used as precursor feed stock material in the conversion of hexagonal boron nitride to cubic boron nitride.